Guests could be asked to dress up in the following ways:1) China theme
This theme would encourage your guests to dress in all manner Chinese. You could make it broader by specifying 'oriental'. Food and decor can be suitably Chinesey. This is a good theme for an Opening Ceremony Party (ie. getting your friends round to watch the opening ceremony).
Specific China costume theme ideas below >>
2) Athlete / Olympic sports theme
This theme would have guests rocking up to your doorstep in Olympic athletic gear - picture a dozen or so people arriving in their trackies and stripping down to their sports gear.
Specific Olympics atheletes theme ideas below >>
3) Countries of the world
This theme would be particularly fitting for an Opening Ceremony party. You could let guests choose which country they want to support, or you can specify particular continents for a guaranteed mix. When their country enters the stadium they have to do something suitably celebratory like get up and dance, skull a drink, run around the room etc
Specific 'countries of the world' theme ideas below.
1) China costume theme ideas